Disk drives comprise a disk media and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and embedded servo sectors. The embedded servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a VCM servo controller to control the velocity of the actuator arm as it seeks from track to track.
FIG. 1 shows a prior art disk format 2 comprising a number of servo tracks 4 defined by concentric servo sectors 60-6N recorded around the circumference of each servo track, wherein data tracks are defined relative to the servo tracks 4. Each servo sector 6, comprises a preamble 8 for storing a periodic pattern, which allows proper gain adjustment and timing synchronization of the read signal, and a sync mark 10 for storing a special pattern used to synchronize to a servo data field 12. The servo data field 12 stores coarse head positioning information, such as a servo track address, used to position the head over a target data track during a seek operation. Each servo sector 6, further comprises groups of servo bursts 14 (e.g., A, B, C and D bursts), which comprise a number of consecutive transitions recorded at precise intervals and offsets with respect to a data track centerline. The groups of servo bursts 14 provide fine head position information used for centerline tracking while accessing a data track during write/read operations.
In the related art, there are free fall sensors (FFS) that are operable to detect a free fall event in response to an acceleration occurring in the disk drive due to the disk drive undergoing a free fall. The disk drive may undergo a free fall, for example, when a user drops the device containing the disk drive. During the free fall, a frequency response of the acceleration signal can be measured, and action can be taken depending on the frequency response. When a free fall is detected, the head is moved off the disk and parked in the ramp to protect against damage.
As disk drives are implemented in smaller and more mobile devices, such as mobile phones and tablets, the possibility of false positives increases for free fall detection. Such mobile devices may be used more vigorously in gaming, exercise or running situations, in comparison to laptops and desktops. If the head is parked during such situations, then the device may be inadvertently disabled by a false trigger of the free fall detection system, even though the mobile device is not in free fall and is being used legitimately. The quality of the free fall detection therefore needs modification to provide timely triggers to initiate the head parking and to reduce the trigger based on false positives (e.g., gaming motion, walking motion, etc.).